Xia Zhang

Formation mechanism and optical properties of InAs quantum dots on the surface of GaAs nanowires.

Formation mechanism and optical properties of InAs quantum dots (QDs) on the surface of GaAs nanowires (NWs) were investigated. This NW-QDs hybrid structure was fabricated by Au-catalyzed metal organic chemical vapor deposition. We found that the formation and distribution of QDs were strongly influenced by the deposition time of InAs as well as the diameter of GaAs NWs. A model based on the adatom diffusion mechanism was proposed to describe the evolution process of the QDs. Photoluminescence emission from the InAs QDs with a peak wavelength of 940 nm was observed at room temperature. The structure also exhibits a decoupling feature that QDs act as gain medium, while NW acts as Fabry-Perot cavity. This hybrid structure could serve as an important element in high-performance NW-based optoelectronic devices, such as near-infrared lasers, optical detectors, and solar cells.

Enhanced absorption of graphene strips with a multilayer subwavelength grating structure

The optical absorption of graphene strips covered on a multilayer subwavelength grating (MSG) surface is theoretically investigated. The absorption of graphene strips with MSG is enhanced in the wavelength range of 1500 nm to 1600 nm by critical coupling, which is associated with the combined effects of a guided resonance of MSG and its photonic band gap effect. The critical coupling of the graphene strips can be controlled by adjusting the incident angle without changing the structural parameters of MSG. The absorption of graphene strips can also be tuned by varying key parameters, such as grating period, strip width, and incident angle.

Growth of InAs quantum dots on GaAs nanowires by metal organic chemical vapor deposition.

InAs quantum dots (QDs) are grown epitaxially on Au-catalyst-grown GaAs nanowires (NWs) by metal organic chemical vapor deposition (MOCVD). These QDs are about 10-30 nm in diameter and several nanometers high, formed on the {112} side facets of the GaAs NWs. The QDs are very dense at the base of the NW and gradually sparser toward the top until disappearing at a distance of about 2 μm from the base. It can be concluded that these QDs are formed by adatom diffusion from the substrate as well as the sidewalls of the NWs. The critical diameter of the GaAs NW that is enough to form InAs QDs is between 120 and 160 nm according to incomplete statistics. We also find that these QDs exhibit zinc blende (ZB) structure that is consistent with that of the GaAs NW and their edges are faceted along particular surfaces. This hybrid structure may pave the way for the development of future nanowire-based optoelectronic devices.

Design and analysis of a dispersion flattened and highly nonlinear photonic crystal fiber with ultralow confinement loss

We present a novel and robust design for a photonic crystal fiber with flattened dispersion, a highly nonlinear coefficient, and low confinement loss for its dual concentric core structure. The proposed fiber has a modest number of design parameters. Analysis results show that the proposed eight-ring photonic crystal fiber is obtained with a nonlinear coefficient greater than 33 W(-1) km(-1) and a near-zero dispersion slope of -7.828 x 10(-4) ps/nm(2)/km at 1550 nm. Ultraflat dispersion with a value between -1.380 and +0.9860 ps/nm/km and a superlow-order confinement loss of 10(-4) dB/km are simultaneously obtained ranging from 1400 to 1625 nm. For practical fabrication, the influence of random imperfections of airhole diameters on dispersion and nonlinearity is discussed to verify the robustness of our design.

Experimental and theoretical investigations on the phase purity of GaAs zincblende nanowires

Interesting phenomena of GaAs nanowire growth have been observed. The nanowires were grown by metal-organic chemical vapor deposition (MOCVD) on GaAs (1?1?1)B substrates with an Au catalyst at 464 ?C. The growth rates of all nanowires were almost the same for a fixed density of Au nanodrops. TEM analysis demonstrates a stacking-fault-free zincblende structure of the nanowires even when their radius is reduced to as small as 12 nm. A theoretical model is developed that is capable of describing the critical radius of zincblende to wurtzite phase transition as a function of vapor supersaturation and material constants. The model shows that the surprising prevalence of the zincblende structure should originate from very high supersaturations during MOCVD.

Growth and photoluminescence of InxGa1−xAs quantum dots on the surface of GaAs nanowires by metal organic chemical vapor deposition

InxGa1−xAs (x = 0.6-1) quantum dots are grown on the {112} side facets of GaAs nanowires by metal organic chemical vapor deposition. The emission spectrum of quantum dots exhibits a multi-peak structure due to size and composition fluctuations. The emission wavelength of quantum dots ranges from 857 nm to 930 nm at 77 K, which is distinctly blueshifted relative to that on traditional GaAs (100) planar substrate. The emission linewidth of an ensemble of quantum dots increases from 29.5 meV to 40.5 meV with increasing the In content, indicating a broader composition distribution of quantum dots.

A New Type Circular Photonic Crystal Fiber for Orbital Angular Momentum Mode Transmission

We proposed a new circular photonic crystal fiber (C-PCF), which can support 14 orbital angular momentum (OAM) modes transmission, with the good features of wide bandwidth, low confinement loss, and all OAM modes at the same size. At 1.55 μm, the designed C-PCF has a very low confinement loss of 3.434 × 10^-9 dB/m for HE41 mode and a relatively low nonlinear coefficient of 3.979 W^-1km^-1 for EH₃₁ mode. The common bandwidth for the four orders of OAM modes is as large as 560 nm (about 1.25 μm-1.81 μm), which does cover all bands of optical fiber communication. Flat dispersion (a total dispersion variation of <;46.38 ps nm^-1 km^-1 over a 750-nm bandwidth from 1.25 μm to 2 μm for TE₀₁ mode) is another feature. With all these good features, the proposed C-PCF could be a well-promising OAM fiber for mode division multiplexing in high capacity fiber communication systems.

Broadband second harmonic generation in GaAs nanowires by femtosecond laser sources

Nonlinear optical property of semiconductor nanowires plays a key role in nanoscale optoelectronics. In this paper, we demonstrate an excellent frequency converter based on GaAs nanowires (NWs), in which second harmonic generation (SHG) is excited by femtosecond lasers from 800 nm to 1800 nm. Simultaneous SHG with a bandwidth of 300-nm is excited by a super-continuum source at 1000–1600 nm. Broadband SHG can also be acquired from an isolated single NW and the process is coherent. The experimental results suggest that GaAs NWs are potential broadband optical nonlinear converters in nanoscale optoelectronics.

Dispersion flattened photonic crystal fiber with high nonlinearity for supercontinuum generation at 1.55 \mum

A robust design for a photonic crystal fiber (PCF) based on pure silica with small normal dispersion and high nonlinear coefficient for its dual concentric core structure is presented. This design is suitable for flat broadband supercontinuum (SC) generation in the 1.55-µm region. The numerical results show that the nonlinear coefficient of the proposed eight-ring PCF is 33.8 W-1.km-1 at 1550 nm. Ultraflat dispersion with a value between -1.65 and -0.335 ps/(nm.km) is obtained ranging from 1375 to 1625 nm. The 3-dB bandwidth of the SC is 125 nm (1496-1621 nm), with a fiber length of 80 m and a corresponding input peak power of 43.8 W. The amplitude noise is considered to be related to SC generation. For practical fabrication, the influence of the random imperfections of airhole diameters on dispersion and nonlinearity is discussed to verify the robustness of our design.

Control of the crystal structure of InAs nanowires by tuning contributions of adatom diffusion

The dependence of crystal structure on contributions of adatom diffusion (ADD) and precursor direct impingement (DIM) was investigated for vapor-liquid-solid growth of InAs nanowires (NWs). The ADD contributions from the sidewalls and substrate surface can be changed by using GaAs NWs of different length as the basis for growing InAs NWs. We found that pure zinc-blende structure is favored when DIM contributions dominate. Moreover, without changing the NW diameter or growth parameters (such as temperature or V/III ratio), a transition from zinc-blende to wurtzite structure can be realized by increasing the ADD contributions. A nucleation model is proposed in which ADD and DIM contributions play different roles in determining the location and phase of the nucleus.